1. Field of the Invention
This invention relates to a totally photonic switch having a pair of D-optical fibers separated by an evanescent coupling region and a pair of thin metal electrodes to which a voltage is applied for causing optical signals to be transferred between the pair of optical fibers in a predictable and controllable manner.
2. Background Art
The totally photonic switch which forms the present invention uses principles that are characteristic of a conventional passive 3 db fiber coupler. Such a conventional fiber coupler is typically fabricated by twisting two identical fibers together under high heat and tension. The tension under heat deforms the fibers to reduce the thickness of their claddings whereby an optical signal is evanescently coupled from one fiber to the other. This twisting of the fibers effectively compresses the length of the coupling or interactive region therebetween so that the coupler can be accommodated according to known packaging techniques, especially in situations where half the optical energy is to be coupled between the fibers.
Doubling the half energy coupling length will permit all of the optical signal to be evanescently coupled, while quadrupling the half energy coupling length will cause the signal to couple completely from one fiber to the other and then completely back to the original fiber. If the evanescent coupling could be controlled (i.e., varied by a factor of 2) over a fixed coupling length, an input optical signal could be switched between two optically coupled fibers.
However, it is difficult to achieve optimal and predictable evanescent coupling in an optical switch by using the conventional technique of twisting together a pair of optical fibers. Firstly, the twisted fibers are bulky and would consume a large area, particularly if a switch network were contemplated using conventional planar semiconductor processing techniques. Moreover, it would be unlikely that the fibers from different couplers could be identically twisted, such that some of the optical switches would have different physical characteristics that vary slightly from one to the other and, consequently, mismatched optical characteristics. What is more, a twisted fiber switch is not electrically controllable, whereby the maximum switching (i.e., coupling) speed would be undesirably limited. In addition, the twisted fiber construction is not compatible with modern photolithographic and microelectronic fabrication processes.
Fiber coupling structures are known in which direct fiber-to-fiber coupling is not possible. Some fiber coupling structures interrupt the fiber path and use a wave guide which correspondingly results in a space consuming fiber-to-wave guide-to-fiber optical path. Other fiber coupling structures require the inefficient use of liquids, mirrors and similar mechanical reflective devices (e.g., including baffles, flexures and the like) which slows the speed in which optical energy can be coupled from one transmission path to another and makes the optical coupling difficult to control. Examples of known optical couplers like those described above are available by referring to one or more of the following Untied States patents:
A totally photonic switch is disclosed for the high speed, efficient fiber-to-fiber coupling of optical signals between a pair of D-shaped optical fibers. A pair of axially aligned troughs are formed in the top and bottom of a semiconductor (e.g., silicon) substrate. The axially aligned troughs are preferably etched in the substrate so as to have a trapezoidal shape and a thin silicon coupling region that is shared by the troughs as a common bottom. The D-fibers are received within respective troughs and laid face-to-face one another against opposite sides of the coupling region so that the cores of the fibers are arranged in close proximity.
The silicon coupling region that is shared by the bottoms of the troughs is completely oxidized to form a thin film silicon dioxide evanescent coupling region extending between the fiber cores. Prior to oxidizing, the silicon coupling region may be doped to an index of refraction that is similar to the cores of the D-fibers. An ultra thin metal film is applied along the top and bottom of the silicon dioxide evanescent coupling region to create a pair of electrodes. By poling the electrodes during fabrication of the switch (i.e., applying a DC voltage to the electrodes at the same time that the semiconductor substrate is heated), the silicon dioxide evanescent coupling region will be polarized so as to become electrooptic. Following fabrication, another DC voltage is applied to the electrodes to selectively control the switch and the coupling of optical energy between the cores of the D-fibers. By applying localized heat, the thin metal film electrodes can also be used to bond the opposing flat faces of the D-fibers to the top and bottom of the silicon dioxide evanescent coupling region. Ultra thin metal films and the aforementioned localized heating can also be employed to bond the D-fibers to the relatively thick silicon dioxide passivation region. As in the case of the thin film silicon evanescent coupling region, the index of refraction of the relatively thick passivation region can be chosen to match that of the cladding of the D-fibers. Accordingly, the cores of the D-fibers received within the axially aligned troughs are separated only by the required cladding thickness along the flat faces thereof, the thin silicon dioxide electrooptic evanescent coupling region running between the flat faces, and the ultra-thin metal electrodes bonded to the top and bottom of the coupling region.
Optical signals are switched between the cores of a pair of the D-shaped optical fibers of a single photonic switch or a plurality of photonic switches arranged on a semiconductor wafer to form a fiber coupler network. That is, by driving the electrodes which extend along the top and bottom of the evanescent coupling region of the photonic switch to a first DC voltage (e.g., ground), an optical signal is transferred from one of the pair of optical fibers to the other. However, by driving the electrodes of the photonic switch to a second DC voltage (e.g., 3.0 volts), an optical signal is transferred from one of the pair of optical fibers to the other and then back to the first fiber so that the optical signal carried on the first fiber is preserved.